The present invention relates to precoated corrosion-resistant steel pipe piles for marine use which support structures in ports and harbors, seawater and rivers.
1. Description of the Prior Art
Steel pipe piles have been heretofore employed as foundation piles of buildings on land and in harbors, river embankments and piers, and further, with regard to marine structures, have widely been used as steel building materials able to cope with deep water and poor ground.
In general, steel pipe piles for use in harbors, seawater, and rivers have been heretofore used without any covering. In recent years, however, harbor, seawater and river structures are required to possess an extended durability of 40 to 50 years.
It has thus become necessary to take anticorrosion measures enabling steel pipe piles used in such harshly corrosive environments to maintain their corrosion-resistance for such extended periods of time.
Various methods for preventing the corrosion of steel pipe piles have been known, such as use of a coating of tar-epoxy, electrical protection, and a mortar coating method employing fiber reinforced plastic (hereinafter referred to as the FRP cover method).
However, the tar-epoxy coating method is troublesome because it has to be reapplied within a few years. Furthermore, extended corrosion resistance cannot be expected, as even if the steel pipe pile is coated with the tar-epoxy before it is driven into place, the coating is soft and tends to be scored when being handled or driven. In addition, after having been driven into place, it may be struck by driftwood or the like, causing damage to the coating and making the pile more susceptible to corrosion at that point. Moreover, if the steel pipe pile is coated with the tar-epoxy after it is driven into place, it follows that only the part above the water will be protected from corrosion. If the underwater portion of steel pipe pile is to be coated, the cost therefor would become very high because of the necessity of having to drain the water from around the pile.
On the other hand, however, the electrical corrosion protection measure is disadvantageous in that the electrochemical function is such that corrosion protection is difficult in the splash zone and the tidal zone, where steel corrosion develops most rapidly.
Corrosion of steel materials in harbors, seawater and rivers proceeds most rapidly in the splash zone and the tidal zone, and is slower underwater, and slower still in sea mud.
"Splash zone" in this specification refers to the portion above the mean high water mark obtained from the high point of the highest tide; "tidal zone" refers to the zone between the mean high water mark and the mean low water mark; and "seawater zone" refers to the portion below the mean low water mark.
According to a recent study on the corrosion rate of steel structures in harbors conducted by an official organization, the average corrosion rate of steel pipe pile is 0.37 mm/year to 0.6 mm/year in the splash zone, and 0.35 mm/year to 0.5 mm/year in the tidal zone and thereabout. It was found that the means corrosion rate in the seawater zone tends to gradually decrease as the depth of the seawater increases, and it is less than 0.05 mm/year.
It was also reported that the corrosion rate was 0.1 mm/year to 0.5 mm/year in a riprap layer, 0.05 mm/year in sea sludge, and 0.01 mm/year to 0 mm/year in sea mud.
Assuming a mean corrosion ratio of 1.0 in the splash zone and the tidal zone, the corrosion ratio in the seawater zone amounts only to about 1/10, and to only about 1/50 in the sea mud.
It follows from the above that electrical corrosion protection is hardly effective for the steel pipe pile in the splash zone and the tidal zone where protection against corrosion is most desired. Consequently, as a most effective means for preventing corrosion in the splash and tidal zones, where corrosion is most marked, the following method has recently been proposed.
FIG. 1 of the accompanying drawings shows an embodiment of a conventional method of preventing corrosion.
In FIG. 1, in a steel pipe pile 1 driven into the sea bed 3, a FRP tubular cover 9 encloses the splash zone 4, the tidal zone 5, and the part of the outer surface of the pile just below the tidal zone 5, with the space between the tubular cover 9 and the steel pipe pile 1 being filled with mortar 10. The lower end of the mortar 10 is covered by an anticorrosion seal means 11. In FIG. 1, 7 is a concrete structure and 8 is a riprap layer.
FIG. 2 is an enlarged view of the principal portion of FIG. 1. To carry out the work of FIG. 1, a specialist, such as a diver, is required, and since the work is affected considerably by waves, tides, and other such marine conditions, the method is disadvantageous in that the resultant working efficiency is so poor that there is insufficient waterproofness along the boundary between the concrete structure built onto the top of the steel pipe pile and the mortar filling. Moreover, the cost is high.